The present invention relates to display devices, and more particularly to driving an active matrix electrophoretic display by varying a common voltage.
Displays, such as liquid crystal (LC) and electrophoretic displays include particles suspended in a medium sandwiched between a drive or pixel terminal and a common terminal. The pixel terminal includes pixel drivers, such as an array of thin film transistors (TFTs) that are controlled to switch on and off to form an image on the display. This conventional method of driving a display is referred to as scan line driving. The voltage difference (VEink=VCE−Vpx as shown in FIGS. 3 and 5A) between a TFT or the pixel terminal 101 and the common terminal 102, which is on the viewer's side of the display, causes migration of the suspended particles, thus forming the image. Displays with an array of individually controlled TFTs or pixels are referred to as active-matrix displays.
In order to change image content on an electrophoretic display, such as from E Ink Corporation for example, new image information is written for a certain amount of time, such as 500 ms to 1000 ms. As the refresh rate of the active-matrix is usually higher, this results in addressing the same image content during a number of frames, such as at a frame rate of 50 Hz, 25 to 50 frames. Electrophoretic active matrix displays are applied in many applications including, for example, e-readers. Although this text refers generally to E Ink as examples of electrophoretic displays, it is understood that the invention can be applied to electrophoretic displays in general, such as for example SiPix where the microcups are filled with white particles in a black fluid.
Circuitry to drive displays, such as electrophoretic displays, is well known, such as described in U.S. Pat. No. 5,617,111 to Saitoh, International Publication No. WO 2005/034075 to Johnson, International Publication No. WO 2005/055187 to Shikina, U.S. Pat. No. 6,906,851 to Yuasa, and U.S. Patent Application Publication No. 2005/0179852 to Kawai; U.S. Patent Application Publication No. 2005/0231461 to Raap; U.S. Pat. No. 4,814,760 to Johnston; International Publication No. WO 01/02899 to Albert; Japanese Patent Application Publication Number 2004-094168, and WO2008/054209 and WO2008/054210 to Markvoort, each of which is incorporated herein by reference in its entirety.
The grey level of a pixel will be referred to as the pixel state P and its value is measured e.g. by the reflectivity of the pixel. The pixel state P of a pixel in an electrophoretic display remains stable when the driving voltage differential VEink is switched off, i.e. VEink=0V. The pixel state P can be anywhere on a grey scale between the two extreme pixel states of the pixel, such as for example black and white. This pixel state stability in the absence of driving voltage is an advantage, as it means that power is only required during a display update. However, the disadvantage is that driving an electrophoretic display is complicated: in order to drive the display one has to know the current pixel states and the intended new pixel states of the display. Typically a so-called Look Up Table (LUT) is used wherein, for example for 16 grey levels this LUT provides 16×16 waveforms or scan line driving values, giving a recipe for a pixel to be driven from each of the 16 possible grey scales to each of these 16 grey scales.
Making a LUT is complicated because the uniformity of the grey levels, for example the percentage of reflectivity of the pixels, must be assured. The difference between grey levels must be equidistant in reflectivity independent of the current image (image history) and independent of the new image (crosstalk). Non-uniformities in TFT backplane and electrophoretic front plane make this problem more imminent. There is the need for an update that is as short as possible. Accordingly, there is a need for better displays, such as displays that tackle the complications of making a satisfactory LUT and provide a more uniform and reliable image update. Additionally there is a need to conserve energy and minimize stresses caused, for example, by voltage differences across various parts of the circuitry, such as the column-row crossings.